The present invention is directed to an improved saddle frame for a single-axle truck of a railway freight car, such as that manufactured by National Castings, Inc., under the name of "Unitruck II." In using such a single-axle truck for a railway freight car, it has been discovered that hot-box detection of the wheel bearings of the axle is not adequately carried out because of the projection of the lower flanges of the saddle frame's center web section toward the inboard side thereof, a distance such that the infrared rays from the wheel bearing to the infrared ray-detecting hot-box detector are blocked. Hot-box detectors are typically mounted on the inboard side of the truck side frame on the rail bed adjacent but outboard of the rail thereof, and sense an infrared beam emanating from the wheel bearing in order to determine the amount of heat given off by the respective wheel bearings, in order to determine the state of operating temperature thereof, to decide whether or not the bearing functions satisfactorily. With the conventional single-axle truck, the inboard sections of the flanges project inboardly to such a degree that these infrared rays, which are typically detected at an angle of 45 degrees, are intercepted by the inboard side of the lower flanges to thereby prevent their reaching the hot-box detector from the wheel set and wheel bearing.